1. Field of the Invention
This invention relates to a method of manufacturing semiconductor devices from a continuous length of leadframe. It more particularly relates to a method of producing semiconductor devices of the type wherein each semiconductor chip is sandwiched between at least one pair of leads for bonding and encapsulated hermetically in a molded resinous body. The invention also relates to a leadframe and a differential overlapping mechanism for advantageously carrying out the method.
2. Description of the Prior Art
As is well known, elongated or continuous leadframes are used for quantity production of semiconductor devices, such as diodes, transistors, and so on. Various attempts have heretofore been made to improve the productivity of such a manufacturing process and to simplify the process in addition to enhancing the quality of the products.
Two typical prior art methods of manufacturing semiconductor devices are now described with reference to FIGS. 15 to 18 of the accompanying drawings for the convenience of explanation.
In one typical prior art method shown in FIGS. 15 to 17, a pair of parallel leadframes A, B respectively formed with aligned leads 2', 3' at a constant pitch P' are transferred longitudinally thereof, as indicated by the arrows. During such transfer, semiconductor chips 1' are bonded to the respective leads 2 of one leadframe A, as shown in FIG. 16. Subsequently, the pair of leadframes A, B are brought closer to each other, so that the leads 3' of the other leadframe B is overlapped on the chips 1' for bonding thereto, as shown in FIG. 17. Finally, each chip 1' together with the associated leads 2', 3' is encapsulated in a molded resin body 4' which is cut off from the respective leadframes A, B to provide an individual semiconductor product.
However, the above prior art method is disadvantageous in that two separate leadframes A, B are required for production of semiconductor devices, thereby posing handling inconvenience (e.g. requiring two separate transfer guides, and etc.) in addition to involving material waste. Further, because of the separate nature of the leadframes A, B, the respective leads 2, 3, which are kept free as projecting, are likely to come into contact with external objects particularly when the leadframes A, B are separately handled. As a result, the leads may be subjected to unacceptable deformation during the manufacturing process, consequently resulting in a higher chance of quality deterioration.
According to another prior art method shown in FIG. 18 of the accompanying drawings and disclosed for example in Japanese Patent Application Laid-open No. 62-35549 (Laid-open: Feb. 16, 1987; Application No.: 60-176036; Filed: Aug. 8, 1985; Applicant: Mitsubishi Electric Corp.), an integral leadframe C is used which includes a pair of longitudinal bands C1, C2 connected together by bar-like connecting segments C3 spaced longitudinally of the leadframe at a constant pitch P". One longitudinal band C1 is formed with a group of longer leads 2" which are spaced longitudinally of the leadframe and extend toward the other longitudinal band C2, whereas the other band C2 is formed with a group of shorter leads 3" which are also spaced longitudinally of the leadframe but extend toward the one longitudinal band C1 in staggered relation to the group of longer leads 2".
In manufacture, the integral leadframe C is transferred longitudinally thereof, as indicated by an arrow. During such transfer, a semiconductor chip 1" is bonded to each shorter lead 3". Subsequently, a corresponding longer lead 2" is bent along a bending line C4, so that the bent portion is overlapped on a corresponding chip 1". Finally, the chip together with the associated leads is hermetically molded in a resin body 4", and cut off from the leadframe to provide an individual semiconductor product.
Compared with the method shown in FIGS. 15 to 17, the method of FIG. 18 is advantageous in that semiconductor devices can be produced from the single leadframe C which enables easier handling. Further, all of the leads 2", 3" are always located in the same plane of the leadframe throughout various stages of the manufacturing process. Thus, the longitudinal space between the two longitudinal bands C1, C2 is an area always protected by these bands, so that the leads 2", 3" arranged in this protected area are less likely to come into contact with external objects. Therefore, it is possible to avoid unexpected deformation of the leads, thereby improving the quality of the final products.
However, the prior art method of FIG. 18 is also disadvantageous in various points. First, the bent portion of each longer lead 2" causes a weight increase of the individual semiconductor device in addition to involving material waste. Second, each pair of leads 2", 3" are spaced longitudinally of the leadframe C before and after the molding operation, so that the resulting product must necessarily have an increased width S which also causes a weight increase. Third, this width increase also results in decrease in number of the products obtainable per unit length of the leadframe C, thus causing material waste.
More importantly, the bending of the longer leads necessitates a slowdown in the transfer speed of the leadframe. Combined with the width increase of the individual products (therefore a length increase of the leadframe), such a slowdown greatly reduces the productivity of the manufacturing process, consequently resulting in a cost increase.